Structures and Reactivities of Cocrystals Involving Diboronic Acids and Bipyridines: In Situ Linker Reaction and 1D‐to‐2D Dimensionality Change via Crystal‐to‐Crystal Photodimerization

Abstract Cocrystallizations of diboronic acids [1,3‐benzenediboronic acid (1,3‐bdba), 1,4‐benzenediboronic acid (1,4‐bdba) and 4,4’‐biphenyldiboronic acid (4,4’‐bphdba)] and bipyridines [1,2‐bis(4‐pyridyl)ethylene (bpe) and 1,2‐bis(4‐pyridyl)ethane (bpeta)] generated the hydrogen‐bonded 1 : 2 cocrystals [(1,4‐bdba)(bpe)2] (1), [(1,4‐bdba)(bpeta)2] (2), [(1,3‐bdba)(bpe)2(H2O)2] (3) and [(1,3‐bdba)(bpeta)2(H2O)] (4), wherein 1,3‐bdba involved hydrated assemblies. The linear extended 4,4’‐bphdba exhibited the formation of 1 : 1 cocrystals [(4,4'‐bphdba)(bpe)] (5) and [(4,4'‐bphdba‐me)(bpeta)] (6). For 6, a hemiester was generated by an in‐situ linker transformation. Single‐crystal X‐ray diffraction revealed all structures to be sustained by B(O)−H⋅⋅⋅N, B(O)−H⋅⋅⋅O, Ow−H⋅⋅⋅O, Ow−H⋅⋅⋅N, C−H⋅⋅⋅O, C−H⋅⋅⋅N, π⋅⋅⋅π, and C−H⋅⋅⋅π interactions. The cocrystals comprise 1D, 2D, and 3D hydrogen‐bonded frameworks with components that display reactivities upon cocrystal formation and within the solids. In 1 and 3, the C=C bonds of the bpe molecules undergo a [2+2] photodimerization. UV radiation of each compound resulted in quantitative conversion of bpe into cyclobutane tpcb. The reactivity involving 1 occurred via 1D‐to‐2D single‐crystal‐to‐single‐crystal (SCSC) transformation. Our work supports the feasibility of the diboronic acids as formidable structural and reactivity building blocks for cocrystal construction.


Introduction
Self-assembly is a ubiquitous process in Nature that enables systems to adapt to environmental changes. [1] At the molecular level, adaptation originates from weak, non-covalent interactions and is subject to supramolecular synthons encoded in molecular structures and the process of molecular recognition. [2][3][4][5][6][7] In this context, boronic acids are highly versatile building blocks to develop functional supramolecular materials (for example, saccharide sensors, [8] pharmaceutics, [9][10][11][12] porous, [13] and photoactive solids [14,15] ). The acid moiety, for example, has a capacity to accommodate conformational changes in the B(OH) 2 group (that is, syn-syn, anti-syn, anti-anti) upon hydrogen-bond mediated self-assembly. [16][17][18] The acid group also exhibits a preference to function as a DD (where: D = hydrogen-bond donor) module for molecular recognition with appropriate AA moieties (where: A = hydrogen-bond acceptor). [19,20] Collectively, the conformational flexibility and hydrogen-bonding capabilities allow for hydrogen-bonded substrates to be assembled and organized in close proximity in the surrounding environment of the acid. [21] While conformational landscapes of boronic acids in the solid state are now becoming established, information concerning the self-assembly of diboronic acids as related to hydrogen-bond mediated self-assembly has yet to be extensively addressed. [20] In this context, reports by Bonifazi et al. exploit selfadapting capacities of diboronic acids to achieve supramolecular polymers [19] and discrete assemblies as DD units through H-bonding with AA N-acceptors. [20] The studies demonstrate diboronic acid recognition in the presence of complementary acceptors. The work follows earlier studies by Pedireddi and Höpfl involving cocrystals of monoboronic and diboronic acids with 4,4'-bipyridine (4,4'-bpy) and 1,2-bis(4pyridyl)ethylene (bpe). [22,23] Given our recent studies involving cocrystals of monoboronic acids with N-acceptor bipyridines, [24,25] we sought to explore the capacity of changes to the structures of diboronic acids [that is, 1,3-benzenediboronic acid (1,3-bdba), 1,4-benzenediboronic acid (1,4-bdba) and 4,4'-biphenyldiboronic acid (4,4'-bphdba)] and bipyridine coformers [that is, alkane vs. alkene groups of bpe and 1,2-bis(4-pyridyl)ethane (bpeta)] to result in conformational and assembly variations of the B(OH) 2 groups. The variations are expected to trigger adaptability of boronic acids in the self-assembly process (Scheme 1). [22] Our work involving cocrystals of boronic acids is also motivated by ongoing interests to direct photodimerizations in organic solids and, more generally, to learn how principles of supramolecular chemistry can affect and control chemical reactivity in the crystalline state. We identified monoboronic acids to function as hydrogen-bond-donor DD templates that direct intermolecular [2 + 2] photodimerizations of bpe to generate tpcb. After photoreaction, the resulting cyclobutane tpcb functions as an AA acceptor as evidenced by single-crystal reactivity properties. [14] Monoboronic acids can now be regarded as tools to control photocycloaddition in the solid state, although efforts are necessary to probe the scope of the selfassembly process and templating behavior. We have also demonstrated the ability of related boronic esters to serve as templates of monopyridines in the form of stilbazoles. [26] Here, we report the self-assembly of diboronic acids and the extended bipyridines bpe and bpeta showing that subtle changes to constituent components lead to broad diversification of cocrystal composition, supramolecular architectures, crystalline motifs, and reactivities of components in both the solid state and liquid phase. We show linear 1,4-bdba to display all syn-syn and all anti-anti conformations in the 1 : 2 cocrystals [(1,4-bdba)(bpe) 2 ] (1) and [(1,4-bdba)(bpeta) 2 ] (2), respectively, generating 1D chains (Scheme 2a and b). For angular 1,3-bdba, the cocrystals exhibit hydrated 1 : 2 : 2 and 1 : 2 : 1 supramolecular assemblies [22] where the components of [(1,3-bdba)(bpe) 2 [27] generates the cyclobutane derivative tpcb stereoselectively and in quantitative yield. For 1, the reaction in the solid state occurs in a single-crystal-to-single-crystal (SCSC) transformation wherein the dimensionality of the polymer undergoes a 1D-to-2D change to generate [(1,4-bdba)(tpcb)] (1R). In the photoreacted solid, the photoproduct tpcb interacts with four different diboronic acids.

Results and Discussion
Our original aim was to compare structural hydrogen-bonding motifs containing diboronic acids (1,3-bdba, 1,4-bdba and 4,4'bphdba) and bipyridines (bpeta and bpe). The ditopic hydrogen-bond-acceptors were selected based on structural effects that arise owing to the different hybridizations of the central carbon-carbon linkage (sp 3 vs. sp 2 ). To synthesize the cocrystals, the corresponding diboronic acid was placed in a vial with methanol and acetone as solvents. Each bipyridine was then added in a 1 : 2 stoichiometry and the solution stirred for approximately 10 min at room temperature. Crystals suitable for single-crystal X-ray diffraction (SCXRD) analysis were achieved by slow solvent evaporation (Figures S1-S7, see Supporting Information). Relevant crystallographic data are summarized in Tables S1-S7. Phase purities were established by comparisons of calculated and experimental powder X-ray diffraction analysis (PXRD) patterns (Figures S8-S13, see Supporting Information).
The solid products were also characterized by 1 H NMR spectroscopy in solution (Figures S15-S20, see Supporting Information).

Chemistry-A European Journal
Research Article doi.org/10.1002/chem.202104604 the bpe molecules assemble as face-to-face π-stacked dimers within the 1D structure [centroid-centroid 3.59 Å and N···N 3.527(4) Å]. The conformation is akin to a DD hydrogen-bond donor module. [19] The B(OH) 2 groups lie coplanar (0.08°) with the pendant aromatic ring while the bpe and 1,4-bdba rings are twisted toward orthogonal (64°). The pyridyl rings of bpe are approximately coplanar (twist angle~5.73°). The stacking of bpe generates infinite face-to-face π stacks with the combined hydrogen bonding and stacking producing a 2D arrangement within the bc-plane (Figure 1a). The C=C bonds of bpe lie disordered over two sites (occupancies: 0.45/0.55 at temperature 298 K). The stacked alkenes are parallel with the C=C bonds separated by 3.80 Å and 3.91 Å, which is a geometry suitable for an intermolecular [2 + 2] photodimerization. [27]

Chemistry-A European Journal
Research Article doi.org/10.1002/chem.202104604 When single crystals of 1 were subjected to UV irradiation for 3 h (450 W, medium-pressure Hg-lamp), bpe reacted to form [(1,4-bdba)(tpcb)] (1R) in quantitative yield as confirmed by 1 H NMR spectroscopy ( Figure S21; see Supporting Information). The formation of tpcb was evidenced by the disappearance of the olefinic signals (7.54 ppm) and appearance of cyclobutane signals (4.66 ppm). Visual examination suggested the single crystals to undergo a SCSC photoreaction.
A SCXRD analysis of the photoreacted solid 1R revealed components of the cocrystal to undergo a SCSC transformation.
The components occupy the same monoclinic space group C2/m, with the asymmetric unit consisting of a quarter molecule of 1,4-bdba and a quarter molecule of tpcb. The cyclobutane lies disordered on a two-fold rotation axis (occupancies: 0.45/ 0.55) ( Figure S2, see Supporting Information). The cell volume of 1R increased on the order of 1.6 % versus the unreacted solid.
The photodimerization resulted in a structural modification of the DD recognition properties of the À B(OH) 2 group. Specifically, the photoreaction involved the larger of the stacked C=C bond separations (that is, 3.91 compared to 3.80) (Figure 1). [28] As a result, each À B(OH) 2  The SCSC transformation, thus, involved a 1D-to-2D change in dimensionality of the hydrogen bonds. [29] To our knowledge, 1R is the first example of a hydrogen-bonded architecture based on a diboronic acid that undergoes a change in dimensionality. There is, thus, a need to consider changes in dimensionally that can occur when engineering topochemically reactive alkenes in solids based on the À B(OH) 2 group. The observations also attest to the conformational flexibility of the À B(OH) 2 group.

Chemistry-A European Journal
Research Article doi.org/10.1002/chem.202104604 bpe, a half molecule of 1,3-bdba and one molecule of water ( Figure S4, 2 ]. [22] The acid adopts an all syn-syn conformation with the B(OH) 2 groups approximately coplanar with the aromatic ring (distortion angle: À 3.2(5)°for C2 À C1 À B1 À O1). The pyridyl rings of bpe deviate from coplanarity (twist anglẽ 9.8°). As a consequence of the assembly, the bpe molecules form infinite face-to-face π···π stacks with nearest-neighbor C=C bonds separated on the order of 3.67 Å, which satisfies the geometry criteria for a [2 + 2] photodimerization [27] (Figure 3a and b). When irradiated with UV-radiation, the cocrystal 3 was determined to be photoactive to generate tpcb in quantitative yield ( Figure S22; see Supporting Information). We note that after the photoreaction, the PXRD diffractogram exhibited significant broadening ( Figure S14; see Supporting Information) excluding a SCSC transformation in this case.
The 4,4'-bphdba and bpeta components also interact by CÀ H···π contacts (C···π 2.981-3.034 Å) along c (Figure 6c). We are unaware of a case wherein a boronic acid has undergone an in-situ transformation to form a cocrystal. The in-situ reaction effectively serves to restrict the dimensionality of the selfassembly process relative to 5. We note that in-situ linker transformations are reported for metal-organic frameworks (i. e., MOFs) and materials. [32] The one-pot reactions have been shown to be useful to generate unusual organic ligands that are a challenge to access in the generation of novel MOF materials. In the current study, the in-situ formation of the hemiester may provide a unique avenue to construct novel organic cocrystals based on diboronic acids.

Conclusion
We have reported a comparison of structural and reactivity aspects concerning cocrystals and hydrogen-bonded frameworks involving diboronic acids and bipyridines. The comparative analysis reveals that the coformers with the diboronic acids play an important role to support adaptability of the diboronic acids in the self-assembly process. UV radiation of single crystals of 1 resulted in the 1D-to-2D crystal-to-crystal [2 + 2] photodimerization. Our work also shows the formation of hemiester 6 by in-situ linker transformation. We expect our results to impact the further development of polymeric assemblies based on boronic acids in organic solids and as related to field of crystal engineering.